The out blooming of information explosion as well as the rapid expanding of the worldwide networking applications naturally trigger the growing demand for channel bandwidth for data and signal transformation. The 10 Gbps Ethernet Standard for optical networking was announced in 2002. The major key to the current technology development and compatibility effort focuses on methods for fast and efficient information retrieval and transformation. The highly advanced information technology development countries in Europe, the United States, and Japan, all gear on aggressive research and development effort to achieve mass data information transformation via high speed optical networking as the foundation for information high way development.
Under the trend of high speed optical networking/complete optical networking, the distance for efficient transmission has also been expanded. Other than decreased number of the conventional electrical networking nodes, the functional monitor on an electrical networking node also has to gradually disappear by itself. This naturally directs the trend for networking development into technologies providing a complete optical network with the functional monitor capability in a traditional electrical network. The quality of an optical signal is evaluated through the measurements of the power, wavelength, optical signal-to-noise ratio (SNR), etc., of the optical signals. For digital transmission systems, especially, two key measurements, the bit error rate (BER) and the Q-factor, are commonly evaluated for functionality index to more accurately monitor the quality of optical signals.
The detection for the bit error rate requires more complicated equipment, and the time required for detecting a lower BER takes even longer. Taking the example for transmitting signals at 10 Gbps, the time for detecting a BER value which is smaller than 10E-15, is 27 hours. For bulky video/audio signals, the BER measuring is hard to effectively perform the real-time quality monitoring to discover/prevent problems. Under such circumstances, the Q-factor measuring, with the advantage of being theoretically equivalent BER, which can be evaluated via speedy measurement, can thus replace BER as a reliable parameter for the system functionality assurance. While BER measuring requires receiving all signals and can only be able to perform at the receiving end, the Q-factor measuring has the advantage of performing the evaluation at any communication node in the signal transformation routing, without the need to stop transmitting signals. Compared with BER measuring, the Q-factor measuring provides more flexibility and is more suitable for the quality assurance for the real-time video/audio signal transmission system.
The definition of Q-factor is as follows:
      Q    =                            m          1                -                  m          0                                                  σ            1                    +                      σ                          0              ⁢                                                                                  ⁢                                        ,wherein, the m1, m0 represent the measured mean values, while the receiving optical signals are 1 and 0, respectively. σ1, σ0, on the other hand, represent the measured standard deviations, while the receiving optical signals are 1 and 0, respectively. This is illustrated in FIG. 1.
With a Gaussian distribution for the noise probability and the inter symbol interference (ISI) meets the condition for ignorance, the relationship between the Q-factor and the BER at the receiver end can be expressed as follows:
  BER  =                    1        2            ⁢              erfc        ⁡                  [                      Q                          2                                ]                      ≈                            exp          ⁡                      (                                          -                                  Q                  2                                            /              2                        )                                    Q          ⁢                                    2              ⁢                                                          ⁢              π                                          .      The graphics for the above relationship is shown in FIG. 2.
In general, the Q-factor measurement needs sampling mechanism to detect the optical power variation of the input signals at the 1/0 levels, that's why it needs sampling pulse to trigger the sampling process. Higher speed of signal transmission needs narrower time period of the sampling pulses. Using the 10 Gbps signals as an example, the corresponding time period of the sampling pulse should be at least smaller than 100 nano seconds. Generating narrower time period of sampling pulses for increased transmission speed is complicated in both optical domain and electrical domain.
U.S. Pat. No. 6,720,548 disclosed a Q-factor monitoring technique which utilizes multiplexed measuring object light and sampling light to enter a nonlinear optical crystal and do the sampling process in the optical domain. This process, however, is very expensive for its high powered optical sampling pulses.
Another U.S. Pat. No. 6,430,715 disclosed an alternative Q-factor monitoring technique by using two detecting circuits to determine the best threshold. Since this technique needs to pre-convert the optical signals back to electrical signals, it requires an expensive high-frequency photo detector.
In an article entitled “Simple Measurement of Eye Diagram and BER Using High-Speed Asynchronous Sampling” in the IEEE Journal of Lightwave Technology, vol. 22, pp. 129-1302, May 2004, Ippei Shake, Hidehiko Takara, and Satoki Kawanishi proposed an asynchronous sampling technique. FIG. 3 shows the schematic diagram of its Q-factor monitoring system. In this system, the internal pulse is a 1 GHz repetitive electrical pulses using electrical absorption modulator as an electro-optic sampling module 303. The electrical pulse generator 301 and the electro-optic sampling module 303 are smaller and simple compared with the previous conventional sampling system modules. The corresponding optic-electro converter 305 does not have the restriction, in which the bandwidth has to be greater than the speed of the signal transmission compared with the previous conventional sampling systems.
In this technique of the article, the sampling speed is slower than the transmission speed of signals. The resulting asynchronous sampled Q-factor value from the analytical sampling of the signal processing circuit 307 of the system is called Qt. The average of all Qt. values is called Qavg. In order to obtain the sampling speed of the internal circuit, the signal transmission speed has to be predetermined, such as SONET/SDH, Ethernet, etc. Those are with different transmission standards. If the system is not using the clock fetch technology, the signal frequency cannot be accurately obtained. Therefore, the sampling frequency for the internal circuit has to be independently determined. In other words, this process also implies possibly increased measurement deviation.
There also are some related techniques which use random factor to determine the sampling frequency for the internal circuit. This type of techniques, however, still needs some predefined threshold values for the levels 1 and 0 to determine the data ranges for the target statistics. This approach of measuring technique results in even greater data deviation.
In summary, the foregoing related conventional techniques for optical signal sampling, the asynchronous sampling technique although is simpler, but the requirement for meeting the characteristic regulation for the electrical absorption/electro-optic modulator sampling module is also pretty strict. With even higher transmission speed, the electrical characteristics of the signal processing circuit seriously affects the quality of the optical signal measurement.